STATIC LIGHT-SCATTERING-STUDIES ON XANTHAN IN AQUEOUS-SOLUTIONS

A commercial sample of xanthan was (i) sonicated and fractionated by G PC on Sepharose C1-2B with 0.1 M NaCl as eluant/solvent. The angular d ependence of scattered light was measured (offline) and interpreted in terms of the wormlike chain model using the ''master curve procedure' '. The persistence length was estimated as L(P) approximate to 150 nm. The mass per unit length M(L) approximate to 2000 g/mol . nm indicate s double-stranded chains. (ii) The parent sample was studied at variou s ionic strengths I (0.01 mol/L less than or equal to I less than or e qual to 0.3 mol/L). The conditions for optimum solubility and clarific ation (ultracentrifugation/membrane filtration) were checked. At highe r ionic strength, the sample was only partly soluble and the solution contained supermolecular structures even after clarification. Good sol ubility was observed in 10 mM NaCl. The Zimm procedure was used to obt ain the average molar mass M(W), the second virial coefficient, and th e radius of gyration R(g). The interpretation of the scattering functi on led to M(W) = 2.88 x 10(6) g/mol, R(g) = 241 nm, L(P) = 149 nm, a p olydispersity sigma(M) = 0.5, and double-stranded chains. At room temp erature, solutions revealed a tendency for aggregation, which could be prevented by storage at elevated temperature.